The Associated Press's official Twitter account, @AP, issued this tweet (it's since been removed):

Hacked @AP Tweet: via Buzzfeed

I was in the bathroom reading Twitter (I know.) when this first
broke, and the first thing I saw wasn't one of the most respected news
sources in the world tweeting that the White House had been bombed. I
don't follow the AP on my personal account, so the first thing I saw was
this:

The next three tweets I saw about this possibly breaking news story were
as follows: "there's no way that @AP tweet is real," "Not believing
this," and "h a c k t i m e." Not a single person in my feed believed
the tweet; the closest was Anil Dash asking for "other sources." The jokes followed immediately--jokes about
those who had bungled coverage of the Boston Bombing (the New York
Post, CNN, former Reuters social media editor Matthew Keys, Reddit), and
then aggressive ignoring of the tweet. My feed, largely made up of
reporters, editors, writers, and other news-types, barely even bothered
to make a reasoned rejection, so silly was the AP tweet and so jaded our
reaction to news.

That's the same experience everyone had; within seconds, the balance of those talking about the tweet swung from earnest disbelief to cries of "hacked," "fake," and scorn. By five minutes in--an
eternity on Twitter!--the conversation was almost entirely about the "AP
hack," not the "news." Even the stock market, run by alarmist
algorithms, snapped back from an absurd drop in minutes.